The long-term goals of the proposed research are to elucidate the signaling and transcriptional mechanisms regulating neuronal differentiation. We recently discovered that the transcription factor myocyte enhancer factor 2A (MEF2A) promotes postsynaptic dendritic morphogenesis in differentiating neurons in the mammalian brain. In the developing mammalian cerebellum, granule neuron differentiation culminates in the generation of dendritic claws upon which mossy fibers and Golgi neuron axons form connections. Genetic knockdown of MEF2A by RNAi in cerebellar slices and in in vivo in the postnatal rat cerebellum revealed an essential function for MEF2A in postsynaptic dendritic claw differentiation. A transcriptional repressive form of MEF2A that is sumoylated at Lys403 promotes the differentiation of dendritic claws. These findings have raised several fundamental questions on how the novel function of sumoylated MEF2A is regulated in neurons and how sumoylated MEF2A orchestrates postsynaptic dendritic differentiation. To address these questions, we propose to identify the enzyme that stimulates the sumoylation of MEF2A and thereby promotes dendritic claw differentiation. Using candidate and non-biased approaches, we will also identify the gene targets of sumoylated MEF2A that mediate its ability to promote postsynaptic differentiation. Finally, we will characterize the developmental role of a calcium/calcineurin-MEF2A signaling pathway that suppresses MEF2A sumoylation and thereby inhibits dendritic claw morphogenesis. The proposed research represents an important set of experiments that should address a major gap in our understanding of the signaling and cell-intrinsic transcriptional mechanisms that underlie neuronal differentiation. In addition, since postsynaptic dendritic pathology is thought to contribute to the pathogenesis of diverse neurologic and psychiatric disorders, including neurodegenerative diseases and mental retardation, the proposed research should provide the foundation for a better understanding of these disorders.